1. Field of the Invention
This invention relates to electric power switches and in particular to such switches having a monolithic stationary conductor configured to enhance current limiting and resist distortion during closing and high current interruptions.
2. Background Information
Power circuit breakers typically are used as a main breaker in a power distribution system having additional downstream branch circuit breakers. They are also used as transfer switches for switching between alternative power sources, and as network protectors in larger distribution systems. In such systems, the power circuit breaker must have sufficient withstand capability to allow a downstream breaker to respond to a fault in order to minimize the extent of the outage. However, in the instance of a very large fault, such as a fault just downstream of the power breaker, it is desirable to have the power breaker respond promptly to limit the fault current. It is known to provide power circuit breakers with a blow open contact structure for this current limiting purpose. This opening is driven by the electromagnetic repulsion force between the contacts at fault current levels that is enhanced by the current path geometry. Current travels in the stationary line conductor to the contacts and through the moving fingers toward the load conductor. The current paths in the line conductors and the contact fingers are roughly parallel, close beside each other, and electrically out of phase, because at any instant current is traveling in one direction in the one conductor and in the opposite direction in the other conductor. This is commonly called a “reverse loop”. The gap and other parameters of the contract structure, and of the remainder of the circuit breaker, are chosen so that the contacts remain closed at the desired “withstand”, or “threshold”, current level but open very rapidly at high short circuit interrupting current levels. The line conductor must have the mechanical strength to tolerate the high forces from the moving contacts and the high fault currents, and have high current carrying and heat conduction capacity for high continuous current ratings. In addition, an arc runner is needed to provide a smooth transition for an electrical arc to travel off of the contacts and toward the arc chute during interruption. The arc runner should center the arc within the arc chamber over the full range of currents up to a maximum interrupting rating without allowing it to stall on corners or at any abrupt transitions. An arc which is offset severely to one side of the arc chamber can track along the arc chamber wall and fail to enter the art chute plates, resulting in poor interruption performance. Finally, the stationary contact assembly must be mounted rigidly in the circuit breaker housing with accurate positioning of the conductor, contacts, arc runner and other key features with respect to each other, and with respect to the breaker frame and moving contacts and arc chute.
The stationary contact assembly, which includes the arc runner, must be manufactured in a way that has the flexibility to produce the desired geometry cost effectively. The stationary contact assembly for the high-interrupting (current limiting) version of the power circuit breaker should be interchangeable with a standard power circuit breaker contact assembly in the same housing, so that a high interrupting version can be offered cost effectively in the same product family.
Thus, there is room for improvement in current limiting power circuit breakers and particularly in the stationary contact assembly.